Add web UI for random favorite requests

[Jamesllllllllll]

Summary

Add the missing public web quick-request UI for the random favorite request mode from issue #20.

What Changed

• Added the Favorite mode to the public web special-request controls
• Disabled the artist field while favorite mode is selected because the request is queryless
• Wired the web viewer request mutation to submit the queryless favorite request mode

How To Test

1. Open the public playlist page for a channel with saved favorites
2. In the special request controls, select Favorite
3. Confirm the artist input disables and the helper text explains the saved favorites pool behavior
4. Submit the request and confirm it adds a random saved favorite without requiring typed artist input

Screenshots Or UI Notes

This follow-up PR only affects the public web request controls. It adds one Favorite mode button next to Random and Choice, and the artist input is disabled while that mode is active so the action stays clear and minimal.

Checklist

• I tested the change locally
• I updated docs or examples if needed
• I called out any follow-up work or known limitations

Context: PR #80 merged the shared backend, chat, and panel support for issue #20, but the $slug web route file was missed during staging because the shell expanded the path incorrectly. This PR carries only that missing web surface.

[github-actions]

Preview deployment updated.

App: https://request-bot-pr-81.jameskeezer.workers.dev
Frontend worker: request-bot-pr-81
Backend worker: request-bot-backend-pr-81